1. Field of the Invention
This invention relates to optical fiber splicing elements and in one aspect to an improved optical fiber centering and holding device to afford the splicing of and alignment of ends of optical fibers.
2. Description of the Prior Art
The art is becoming replete with devices for centering optical fibers for the purpose of aligning ends of optical fibers for splicing the ends of fibers for continued transmission of light rays along the fibers. The optical fiber used in the telecommunications industry is mostly a single mode glass fiber. This fiber typically contains a 8 .mu.m.+-.1 .mu.m diameter central glass core through which the optical signal is transmitted. The core is surrounded by a glass cladding which has a diameter of 125 .mu.m.+-.3 .mu.m. The cladding has a slightly different index of refraction than the core.
There are a number of prior art devices which have been designed to make permanent connections or splices in single mode glass fiber. To function properly and produce a low loss splice, these devices must align the core portions of the two fiber ends being spliced to within approximately 10% of their core diameter or less than 1 .mu.m.
Some of the prior art devices attempt to achieve fiber end centering and alignment by forcing the fibers into a non-conformable "V" groove or between non-conformable rods. These devices function satisfactorily as long as the fibers being spliced are the same diameter. Examples of such devices are disclosed in U.S. Pat. Nos. 4,029,390; 4,274,708; and 4,729,619.
When the fibers to be spliced have diameters at opposite ends of the tolerance range (122 .mu.m and 128 .mu.m), the non-conformable "V" groove type devices have difficulty providing the required core alignment precision.
Other prior art devices urge the two fiber center lines into alignment with one another by locating the fiber ends between three equally spaced ductile or elastomeric surfaces. These surfaces create an equilateral triangular cross-sectional channel into which the fibers are positioned. The junction between the fiber ends is located usually at the mid point of the channel. These devices provide better performance than the non-conformable "V" groove type devices because they are better able to accommodate differences in fiber diameters. Examples of these devices are found in U.S. Pat. Nos. 4,391,487; 4,435,038; and 4,593,971.
Another prior art patent is U.S. Pat. No. 4,339,172 which utilizes a foldable grip member that is placed within an elastomeric sleeve which affords compressive forces against the grip member to assert pressure on the grip member to grip each of the cables at a number of equally-spaced points around its periphery.
The device of the present invention utilizes three contact surfaces which form a fiber centering channel as do other prior art devices, but the device contains easily definable functional and performance improvements over the prior art. These differences in structure and means of operation are discussed below.
The splicing element of the present invention provides three high precision fiber contact surfaces and at least one of which is movable in relationship to the others on an integral actuation structure affording such movement in a single easy to manufacture part. The fiber clamping surfaces are initially spaced apart to allow fibers to be easily inserted into a progressively narrowing channel or passageway until the end faces are in contact. This structure makes it easy to detect by tactile feedback and/or buckling in the opposing fiber when the first fiber is pushed against the opposing fiber in the passageway. When the fiber ends are in contact, the splice element is closed and the fiber ends are aligned and clamped.
Other prior art devices such as the device of U.S. Pat. No. 4,391,487 and the commercial form sold by the assignee of U.S. Pat. No. 4,391,487 utilizes molded elastomer blocks for centering and aligning the fiber. These blocks contain an essentially triangular cross-sectional passageway which is slightly smaller than the diameter of the fibers. When the fibers are inserted into the molded passageway, they are required to force the elastomer surfaces open by elastically deforming the elastomer contact surfaces. This requires significant compressive force on the fiber to push it into the passageway. Because of this high insertion force, it is often difficult to tell when the fiber end faces are actually in contact inside the splice. Additionally, it is possible to easily break the fibers under compressive buckling as they are forced into the elastomeric centering blocks.
Only very small ductile or elastic deformation of the fiber locating surfaces is required in the splice element of the present invention to provide precise fiber centering and clamping. This deformation creates no external material flow parallel to the axis of the fiber and produces no ductile deformation of the hinged structure around the clamping surfaces. Other prior art devices such as the connector of U.S. Pat. No. 4,435,038 require extremely large deformations of the fiber clamp surfaces and surrounding structure to produce fiber centering and clamping. Unfortunately, these large deformations may result in unwanted distortion of and unequal clamp forces on the fiber engaging surfaces. This in turn may also produce shifting of the fiber center lines and axial separation of the fiber end faces.
Forces high enough to embed the fiber uniformly into each of the clamping surfaces of the splicing element of the present invention by ductile deformation can be generated easily through the hinged lever structure afforded by the legs of the splicing element. Once clamped, the splicing element maintains uniform compressive loads on the fibers which result in high fiber tensile load retention, and excellent thermal cycling stability. The fiber clamp forces are maintained by elastic energy stored in the legs of the splicing element as well as in the clamping structure. Other prior art devices such as the connector of U.S. Pat. No. 4,435,038 tend to relax or elastically recover slightly after removal of the splice crimping tool forces. This relaxation may result in a slight loosening of the fiber in the locating channel which can lead to reduced pull out forces as well as fiber end face separation during thermal cycling.
During the fiber centering and splicing process in the splicing element of the present invention, clamping loads are generated on the fiber. Since glass has very high compressive strength, these loads tend to embed the fibers into the three ductile contact surfaces forming the fiber channel in the element. These clamping loads on the fiber cladding result in excellent tensile load retention capabilities and thermal cycling stability for the splice. In addition, the application of the clamping load is easily and quickly accomplished simply by squeezing together the legs of the splice element. Other prior art devices such as the fiber centering device of U.S. Pat. No. 4,391,487 require adhesives to bond the fiber to the external splice structure beyond the fiber centering blocks. These adhesives can be difficult and time consuming to apply and to cure uniformly.
The splicing element of the present invention has also been designed to permit reopening and release of the fiber ends. If it becomes necessary to reposition fibers within the splice, the clamping force provided on the legs of the element can be easily removed, and the elastic memory in the element legs and hinge sections will cause the fiber locating and clamping surfaces to move apart. The clamping loads on the fibers are then released, and the fibers can be repositioned. Other prior art devices such as the connectors of U.S. Pat. Nos. 4,391,487 and 4,435,038 are permanently crimped or adhesively bonded and cannot be reopened.